Over voltage cooler. In this article, we are going to look at chiller surge. We will examine the causes and diagnostics of chiller surge and understand what happens internally to cause chiller surge.
Scroll down for the Chiller Surge video tutorial on YouTube
During normal operation, refrigerant will leave the compressor through the discharge line and flow to the condenser. From there it passes through the expansion valve and into the evaporator, where it will be sucked back into the compressor through the suction line. The cycle is constantly repeated to allow the chiller to remove unwanted heat from the evaporator, which has collected in the building, and move it to the condenser so it can be sent to the cooling towers.
The compressor is the driving force of the refrigerant between the evaporator and the condenser. The compressor is driven by an electric motor. As it rotates, the internal impeller rotates and draws refrigerant from the evaporator through the suction line. The impeller blades impart angular velocity to the coolant particles, causing them to fly off the tips of the blades in all directions at very high speed.
As the coolant is blown off the turbine blades, it collects in the volute, which is a spiral-shaped channel that surrounds the compressor and expands in diameter along its length. It then passes through the diffuser, where it slows down and converts its kinetic energy to pressure. This pressure builds up in the volute and expels the refrigerant through the discharge line to the condenser.
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| Chiller-section-view- jcool |
In the cutaway view above, you can see the flow path from the evaporator through the suction line to the compressor, which rotates and pushes the refrigerant through the discharge line to the condenser. It flows this way because the compressor causes a pressure difference across the wheel.
The low pressure side is on the evaporator side, where it draws in refrigerant. The high pressure side is after the compressor in the condenser, where the refrigerant is forced through. Pressure builds in discharge line and condenser.
Now each compressor can only supply a certain pressure difference. Every compressor is different and you will need to talk to the chiller manufacturer to find out what a compressor's rating is.
This pressure difference caused by the compressor is known as chiller lift. This is where it takes the low pressure refrigerant and boosts (lifts) it by using the compressor and pushes it into the condenser at a higher pressure.
Example
Let's say this cooler is running at full power. Its maximum lift is from 300 kPa to 900 kPa. (3 bars to 9 bars). This means that it will take refrigerant from the evaporator at a pressure of 300 kPa (3 bar) and compress it and force it into the condenser to its maximum, which is 900 kPa (8 bar).
Therefore, the cooler can provide 600 kPa (6 bar) of lift. (900 kPa – 300 kPa).
Now let's say the pressure in the condenser reaches 1000 kPa (10 bar). Obviously this is much higher than the maximum pressure the compressor can provide. When the pressure in the condenser is higher than the pressure the compressor can apply, a chiller surge occurs.
It is important to note that these numbers are given as an example only. As mentioned, check with your chiller manufacturer to find out what their surge line is and what their chiller riser is.
When the chiller surge occurs, the pressure in the condenser has become too high and the refrigerant begins to back up. This causes it to start flowing in the opposite direction, back through the discharge line and into the compressor.
The compressor will continue to run because its job is to push the refrigerant through the discharge line to the condenser. Since the pressure is already too high, the refrigerant flows back into the turbine blades and out through the suction line to the evaporator.
This is not a good scenario for the chiller. This can cause very serious damage to the cooler. You will know when this is happening because the cooler will make a very loud, screeching noise. This noise will come from the compressor. You will also probably find quite large variations in the number of amps drawn by the compressor.
If you listen to the video, you can hear an example of a cooler surge.
What causes chiller surge?
Well, do we know that there is going to be a pressure increase in the condenser? So what could be causing this then? Most of the time, it usually has something to do with the cooling tower or the condenser water flow between the chiller and the cooling tower. If a partial blockage occurs in this condenser return line, the flow will change. This could cause the condenser water to drop below the required minimum flow rate. This means that the condenser will not be able to remove its heat and this will start to increase the pressure in the condenser.
If the cooling tower cannot reject enough heat, this will also cause a power surge. You will see the temperature of the water rise and this will limit the amount of heat the condenser can absorb. Again, it won't be able to turn down enough heat. This may be because the drive belt broke, or the engine may have a misfire and break.
It is even possible that the distribution of the water inside the cooling tower is altered. For example, there could be a buildup of leaves in the cooling tower. This can reduce the flow of water and create a large enough surface area within the cooling tower to disperse its thermal energy. Therefore, the cooling tower cannot completely reject the unwanted heat and cannot generate a large enough temperature difference in the condenser water line.
Also, the pump filter can become clogged, which will reduce the flow of water through the system.
The distribution pan at the top of the cooling tower can also become clogged with scale and debris, etc. The tubes within the condenser can be coated with scale which would reduce the amount of heat transfer that can occur as this will shift the heat. transfer surface, this is called fouling.
Surges will also occur when the chiller is running at part load, e.g. the cooler runs it lower than its maximum design. Once this load drops to a certain point, it will cause power surges as the temperatures will not be bearable to run the machine.
Troubleshoot chiller surge
Some ways to solve this problem are to install a variable speed drive or a variable frequency drive on the induction motor that drives the compressor. This will change the rotation speed of the compressor to modulate its capacity.
Some chillers may have a built-in hot gas bypass, and this is used to allow capacity to be reduced, while maintaining sufficient gas flow through the compressor.
Some manufacturers will also install variable diffusers on the compressors. This reduces the space in the diffuser through which the refrigerant can flow into the volute, maintaining gas velocity.
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